Toothpaste compositions are well characterised in the literature and many compositions are disclosed in patent specifications and other literature. Toothpaste compositions contain a number of specific components for example abrasive agents, fluoride sources, binders, preservatives, humectants, anti plaque agents, colouring agents, water, flavour and other optional ingredients. Of these components the abrasive agent is required to provide the appropriate cleaning and plaque removal without subjecting the tooth itself to excessive abrasion. Typically a toothpaste composition will contain from about 5% to about 50% preferably up to about 30% by weight of abrasive. Commonly used abrasives are aluminas, calcium carbonates and calcium phosphate. More recently synthetic silicas have been adopted because of their efficient cleaning, compatibility with other ingredients and their physical properties. An important property of a silica for use in transparent toothpaste formulations is its apparent refractive index, and the lower this value in the chosen water/humectant system the more water can be allowed for by the formulator in the transparent toothpaste. The replacement of the more expensive humectant, e.g. Sorbitol, by water leads to substantial cost savings.
The inventors have shown that the apparent refractive index of an amorphous silica can be controlled by the careful selection of process conditions in the preparation of silica. Changing these conditions alters the fundamental features of the overall pore size distribution present in the amorphous silica which in turn determines its apparent refractive index.
In particular, the extent of ultramicropores (below 0.7 nm diameter), which can preferentially adsorb water, when the silica is brought into contact with the humectant/water system, are thought to play a dominant role. Ultramicropores have been classified as pores which are small enough to exclude the nitrogen molecule (Characterisation of Porous Solids: An Introductory Survey--Kenneth SW Sing in Characterisation of POrous Soloids II 1991 Elsevier Science Publishers BV Amsterdam).